1. Field of the Invention
The invention relates to a method for the adaptive filtering of signals as well as an arrangement for the adaptive filtering of signals.
2. Description of the Related Art
The use of adaptive filters is known in digital signal processing, that is, filters that are in a position to adapt their filter coefficients independently to the input signal.
The problem exists, particularly during the use of adaptive filters with so-called echo cancelers, which are used primarily in telecommunications and particularly during voice transmission over networks based on the Internet Protocol (IP)—the so-called Voice over IP (VoIP), that, if narrowband signals such as sinus tones occur during a phase in which the filter is adapted, it leads to erroneous adaptations.
An adaptive filter functions ideally, particularly if a white noise signal is present, since all frequencies occur simultaneously then, and all the information for calculating the coefficients of the adaptive filter is received from the echo path (transmission channel, telephone line). Information for the non-occurring frequencies is lacking with narrowband signals, which can lead to a drifting away of the coefficients.
What is known about this is that the adaptation phase is prolonged until a broadband signal is present once again, which ideally is equal to a white noise, representing an indication of the adaptation's reaching an optimal state.
In the meantime a drifting of the coefficients adapted to the echo, needed for the suppression of the echo thus cannot be prevented.
Furthermore, according to known solutions, the narrowband signals are detected by means of a comparison between the so-called crest factor (peak factor) of a received (voice) signal Rin, which describes the relationship between peak value and effective value of the voice signal, and the arithmetic mean value of the received (voice) signal Rin.
It is a disadvantage that the crest factor is not a clear sign of a narrowband signal, since broadband signals can also generate the same crest factor. In addition, narrowband signals that are not detectable via the crest factor can also occur.